The switched mode power supply (SMPS) is a well-known type of power converter having a diverse range of applications by virtue of its small size and weight and high efficiency. For example, SMPSs are widely used in personal computers and portable electronic devices such as cell phones. An SMPS achieves these advantages by switching a switching element such as a power MOSFET at a high frequency (usually tens to hundreds of kHz), with the frequency or duty cycle of the switching defining the efficiency with which an input voltage is converted to a desired output voltage.
In most SMPS topologies, the output voltage, Vout, is directly proportional to the input voltage, Vin:Vout∝nDVin  Equation 1
In Equation 1 above, D is the duty cycle of the switching, and n=ns/np is the transformer ratio (the number of turns on the secondary side, ns, divided by the number of turns on the primary side, np) if a transformer is used in the SMPS or n=1 if no transformer is used.
The duty cycle is therefore critical in maintaining a high efficiency; a duty cycle of 100% will produce the maximum efficiency.
A number of different control strategies for controlling the duty cycle of an SMPS are known.
One method of control is used in fixed ratio converters or Intermediate Bus Converters also referred to as unregulated converters. These lack all control of the output voltage but run with a maximised duty cycle. This yields maximised power efficiency since the converter transfers energy almost 100% of the time, with the exception of the dead time needed during switching. With this strategy, the output voltage varies with the input voltage according to Equation 1 above. Unregulated converters with different topologies are disclosed for example in U.S. Pat. No. 7,272,021, U.S. Pat. No. 7,558,083, U.S. Pat. No. 7,564,702 and U.S. Pat. No. 7,269,034. Furthermore, narrow regulation of the voltage can be taken care of by second layer SMPSs called Point of Load regulators, this power architecture being referred to as Intermediate Bus Architecture, for example as disclosed in U.S. Pat. No. 7,787,261.
Semi-regulated converters compensate for a varying input voltage (line regulation) at the expense of a varying duty cycle, which reduces power efficiency. Such a converter is disclosed for example in U.S. Pat. No. 7,787,261. The load affects the output voltage and the output voltage decreases with increasing load, also known as droop. Since the output of an SMPS has an LC filter then load transients cause the output voltage to oscillate, and only inherent parasitic resistances dampen the oscillations.
Quasi-regulated bus converters, for example as disclosed in U.S. Pat. No. 7,787,261, are line regulated in only a part of the input voltage range, while in other parts of the input voltage range they are unregulated using 100% duty cycle to maximise efficiency. This yields an increased input voltage range without increasing the output voltage range.
Output regulated converters compensate for varying load conditions and input voltage changes by feedback of the output voltage. Voltage feed forward is often added in order to reduce output voltage disturbances due to input voltage transients. This type of regulation offers the most stable output voltage at the cost of lower efficiency.
Irrespective of the control strategy, it is preferable for the output voltage of an SMPS to remain at its desired level under all conditions. However, transients and changes of the input voltage will cause the output voltage to change almost immediately. This can introduce large changes in the output voltage of the SMPS. Typically, only the inertia in an output filter of the SMPS will decrease this effect.
All the above-described control strategies have drawbacks in terms of output voltage tolerance, transient responses and power efficiency. Furthermore, many of these variables are dependent and optimising one makes the others worse.